As the densities of semiconductor devices increase, technologies capable of providing fine patterns have been developed. When an exposure process is used to form a photoresist pattern, a width of the photoresist pattern can be determined by Rayleigh's equation.
R=k1·λ/NA, where R is a resolution or a minimum distance between resolvable points, k1 is a process parameter, λ is a wavelength of light, and NA is a numerical aperture of lens. According to the above equation, in order to reduce the resolution R, k1 or needs to decrease or NA needs to increase. Exposure processes using extreme ultraviolet (EUV; 13.4 nm) light sources may form patterns smaller than patterns formed using exposure processes using 248 nm KrF or 193 nm ArF excimer lasers as light sources because of a short wavelength. However, the exposure processes using extreme ultraviolet light sources may raise manufacturing costs because an EUV exposure process may require, for example, a vacuum system and a reflection-type photomask.
Accordingly, technologies capable of increasing the numerical aperture NA of lens have also been developed. The numerical aperture NA is proportional to n.sin θ, where n is a refractive index of a medium interposed between the lens and the photoresist. The minimum distance between resolvable points may decrease using the medium having a higher the refractive index. Immersion lithography processes use a medium having a refractive index that is higher than a refractive index of the air and the resolution R thus may increase even if a conventional light source is used. Water having a refractive index of 1.44 may be as the medium of the immersion lithography processes.